Gold nanoparticles have been one of the most extensively studied nanoparticle systems over the past few decades and there are now well established syntheses for a wide range of sizes and morphologies including spheres, nanorods, and nanoplates. See J. Watt et al., Chem. Mater. 27, 6442 (2015); S. E. Lohse et al., Chem. Mater. 26, 34 (2014); L. Chen et al., Nano Lett. 14, 7201 (2014); L. Scarabelli et al., The Journal of Physical Chemistry Letters 6, 4270 (2015); K. Park et al., Chem. Mater. 25, 555 (2013); and A. M. Henning et al., Angewandte Chemie-International Edition 52, 1477 (2013). This high level of research interest is largely due to the possession of well-defined surface plasmon resonances (SPRs) which are sensitive to changes in nanoparticle size, shape and crystallinity. See J. Zheng et al., Nanoscale 4, 4073 (2012); K. Park et al., Journal of Physical Chemistry C 118, 5918 (2014); V. Juvéet al., Nano Lett. 13, 2234 (2013); and H. J. Chen et al., Chem. Soc. Rev. 42, 2679 (2013). The ability to fine tune the optical properties means gold nanoparticles show great potential for a number of applications including theranostics, photothermal therapy, and sensor technologies. See J. Qin et al., Nanoscale 7, 13991 (2015); A. J. McGrath et al., ACS Nano (2015); and Z. Wu et al., Small 8, 2028 (2012).
Metallic gold is chemically inert so gold salts are typically used as precursors for nanoparticle synthesis; tetrachloroaurate (HAuCl4) being the most common. Tetrachloroaurate is highly corrosive, with limited exposure known to cause skin and eye damage. It is hygroscopic and requires a dry atmosphere for storage as well as care in handling to ensure uncontrollable hydration does not affect reaction stoichiometry. It's most damaging aspect however, comes from its production, which requires the dissolution of bulk metallic gold in aqua regia; an incredibly harsh acid. Then, in order to form gold nanoparticles tetrachloroaurate is converted back into Au(0) by a reducing agent; usually sodium borohydride (NaBH4). Although widely used for the synthesis of gold nanoparticles, this reaction is inefficient and highly toxic and violates a number of the 12 Principles of Green Chemistry i.e., the reaction has incredibly poor atom economy (only 1 in 12), and generates environmentally harmful side products, as well as typically using a large excess of reducing agent. There are a number of greener approaches that use environmentally friendly reducing agents; however, they still rely on tetrachloroaurate as the gold precursor and display poor atom economy. See R. K. Sharma et al., J. Chem. Educ. 89, 1316 (2012); S. K. Das et al., Green Chemistry 14, 1322 (2012); and N. N. Dhanasekar et al., J. Microbiol. Biotechnol. 25, 1129 (2015).